1. Field of the Invention
The present invention relates to a voltage to pulse-width conversion circuit, and in particular to a voltage to pulse-width conversion circuit for adjusting the brightness of an electronic display device.
2. Description of the Related Art
Recently, many people enjoy themselves by listening to music programs with audio systems installed for example on the dashboards of their cars. Some of the audio systems have display systems which change displayed information in accordance with the mode selection between cassette mode and the radio mode so as to improve the operability of the audio system. In other words, when a listener listens to a music on cassette tape, the display system displays information with respect to the cassette tape. On the other hand, when the listener listens to a radio program, the display system displays information with respect to the radio program for example mode indication of FM/AM, and frequency of the radio program.
However, the information displayed on the display panel should be clearly viewed in any driving situation, regardless of whether it is daytime or nighttime. To satisfy this requirement, many display panels are provided with, for example, a fluorescent display tube or the like for displaying the information. In addition, they are also provided with a brightness adjustment function for adjusting the brightness of the display in accordance with the surrounding brightness. When the driver listens to a music program while driving the car for a long time from the daytime to the nighttime, if the brightness of the display panel cannot be adjusted, he or she will suffer from eye fatigue which could result in a serious accident. Thus, the brightness adjustment function of the electronic display device, for example a fluorescent display tube installed in the car, is very important.
Related to the brightness adjustment function for adjusting the brightness of the electronic display device to a desired level, a method for controlling the pulse width of a drive signal applied to each segment of a display element of a fluorescent display device and setting the brightness by means of a duty ratio is known. In this pulse width control method, a voltage to pulse-width conversion circuit for converting the pulse width of a brightness control voltage into a predetermined pulse width is used. Many types of conversion circuits have been proposed.
For example, the patent specification of U.S. Pat. No. 4,891,828 discloses the above mentioned voltage to pulse-width conversion circuit. This voltage to pulse-width conversion circuit outputs a pulse width modulated output signal (or a PWM output signal) having a duty ratio set by the operator in the electronic display device and the duty cycle adjusts the brightness of the display device. In other words, this voltage to pulse-width conversion circuit compares a signal S3, where the number of pulses of a reference frequency S2 is counted and then converted into an analog signal, with a PWM control voltage S1, whose amplitude is controlled by the operator, and outputs the resultant signal as a PWM output signal Va.
FIG. 7 shows waveforms of signals representing the relationship among the PWM control voltage S1, the reference frequency S2, the analog signal S3 being converted, and the PWM output signal Va. The signal S3 has stair step voltage values which vary in accordance with the counted values of the reference frequency S2 and which are repeated at predetermined periods. For example, an eight-bit binary counter divides the frequency of the reference frequency signal S2. The output signal with eight bits is input to a D/A converter and then output as the signal S3.
When S1.gtoreq.S3, the PWM output signal Va goes high. When S1.ltoreq.S3 takes, the signal VA goes low. Thus, when a PWM control voltage S1-1 is applied, the voltage to pulse-width conversion circuit outputs a PWM output signal Va-1 as the PWM output signal Va. When a PWM control voltage S1-2 is applied, the circuit outputs a PWM output signal Va-2 as the PWM output signal Va. Consequently, as shown in FIG. 8, the period t during which the D/A converter outputs a predetermined voltage is equal to one period of the reference frequency signal S2. Namely, the period t is constant for all output states of the D/A converter. Thus, the PWM output signal Va varies in steps of period t regardless of the duty ratio of the PWM output signal Va.
Thus, as shown by symbol Q of FIG. 8, when the PWM control voltage S1 becomes equal to the output voltage S3 of the D/A converter, one of the PWM output signals Va-1 and Va-2 shown in the figure is output as the PWM output signal Va in accordance with a delicate variation of the PWM control voltage S1. Thus, the PWM output signal Va becomes-unstable. In particular, when the duty ratio is set to a low value for example 1 to 25%, the instability of this signal becomes significant. In other words, when the PWM control voltage S1 as shown in FIG. 8 is input, since the duty ratio of the waveform Va-1 becomes 1.56% (4/256) and that of the waveform Va-2 becomes 1.95% (5/256), the degree of variation of the duty ratio of the waveform Va-2 against the waveform Va-1 becomes 20%.
Thus, when the voltage to pulse-width conversion circuit in accordance with the related art is used as a control circuit of an electronic display device installed in a car where the noise level is high, noise is superimposed on the PWM control voltage S1 and thereby the voltage S1 delicately fluctuates in the vicinity of the voltage level of the symbol Q. Consequently, the PWM output signal Va varies between the waveforms Va-1 and Va-2. Since the degree of variation of the duty ratio is as high as 20%, when the fluorescent display tube or the like is driven with the PWM output signal Va, a large difference takes place between the brightness of which the tube is driven with the PWM output signal Va having the waveform Va-1 and that driven with the PWM output signal Va having the waveform Va-2. Thus, the difference of brightness becomes a visible flickering. Such a flickering phenomenon is a cause of remarkable degradation of the quality of the display devices of car audio systems.
To prevent such a flickering phenomenon, for example, a method for shortening the variation steps (period t) of the PWM output signal Va is known. To suppress the flickering phenomenon of for example a fluorescent display tube, the degree of variation of the PWM output signal Va should be limited to 10% of the duty ratio. To accomplish this limitation, it is essential to shorten the period t by raising the frequency of the reference frequency signal S2 so as to increase the number of bits of the above mentioned binary counter.
However, when the number of bits of the counter is increased, a D/A converter with the same number of bits should be used. Conventional D/A converters comprise a decoder, analog switches, and voltage dividing resistors whose numbers are the same as the number of outputs of the decoder. When the number of bits of the counter is increased, the numbers of the analog switches and the voltage dividing resistors should be increased.
Thus, when such a voltage to pulse-width conversion circuit is accomplished in an integrated circuit, the size of the chip becomes large thereby raising the cost. In addition, when the duty ratio becomes large, the variation steps of the PWM output signal Va become small. Consequently, when the voltage of the signal S1 is adjusted with a variable resistor, the brightness does not vary unless the variable resistor is largely adjusted.